Currently, there is demand for an image forming apparatus that executes high-speed and high-quality image printing. Accordingly, there is also demand to increase the driving frequency of semiconductor laser elements of a semiconductor laser driver in a laser beam printer or the like and to also increase the number of semiconductor laser elements.
Generally, a laser circuit substrate is made up of a semiconductor laser element and driving semiconductor device which form a semiconductor laser driving circuit (light-emitting circuit), main power transmission wiring, main ground wiring, and wiring which connects the semiconductor laser element to the driving semiconductor device. The light-emitting circuit for outputting a semiconductor laser beam supplies, to the semiconductor laser element, a current of about several tens of mA at a frequency of about 10 MHz to blink the semiconductor laser element, thereby converting a received electrical data signal into an optical data signal. However, when a large current flows through the semiconductor laser driver at a high frequency, noise currents are generated in the light-emitting circuit due to the differences among the impedances of the semiconductor laser elements, driving semiconductor device, main power transmission wiring, main ground wiring, and the wiring which connects the semiconductor laser element to the driving semiconductor device. The generated noise currents propagate to main wiring such as the main power transmission wiring and main ground wiring, and noise currents are also generated even in the main wiring.
Such a noise current deteriorates the quality of a light emitting current supplied from the semiconductor laser element, so demanded high-quality image printing is hindered. If the main wiring is principally connected to a power supply cable, radiant noise is inconveniently produced when the power supply cable acts as an antenna. Furthermore, if the power supply cable shares GND (ground) with other signal cables, radiant noise is inconveniently produced since the noise currents also flow in the other signal cables.
Japanese Patent Laid-Open No. 63-044782 discloses a method of suppressing the occurrence of noise currents which hinder high-quality image printing and generate radiant noise. In this conventional method, a filter is arranged on wiring which connects a semiconductor laser element to a driving semiconductor device. Unfortunately, as driving frequency increases, the deterioration of the light-emitting current waveform caused by filter insertion worsens. For this reason, high-speed, high-quality image printing, and radiant noise reduction cannot be attained simultaneously.
In recent years, to suppress noise currents, a compensation circuit is generally added to a circuit substrate. FIG. 9 shows an example of the circuit substrate with the compensation circuit.
A compensation circuit 10 which is made up of a compensation element 11, wiring 12, and compensation semiconductor device 13 is connected in parallel with a light-emitting circuit 6 from main power transmission wiring 2 made up of a semiconductor laser element 7, wiring 8, and driving semiconductor device 9. With this arrangement, the compensation circuit 10 and light-emitting circuit 6 are complementarily driven, thus implementing a compensation function of allowing a feed capacitor 1 to supply a constant current. Since a constant current flows through the main power transmission wiring 2, noise currents which normally flow through the main power transmission wiring 2 can be suppressed. In addition, noise currents generated in the light-emitting circuit 6 are canceled by a compensation current flowing through the compensation circuit at a branch point 3 of first wiring 4 (4a and 4b) and second wiring 5 (5a and 5b). This makes it possible to suppress propagation of the noise currents to the main wiring side.
When such a constant current driving arrangement is to be adopted, ideally, a wiring form must be designed such that an impedance Z4 of the wiring 4 becomes equal to an impedance Z5 of the wiring 5. The values of the impedances Z4 and Z5 can be equalized by using the wiring 4 and wiring 5 of the same length. Alternatively, the difference between the impedances Z4 and Z5 can be decreased by making their length as short as possible.
However, the position of the semiconductor laser element is determined mainly in consideration of the position of an optical system suitable for processing a laser beam output from the semiconductor laser element. Therefore, the degree of freedom of wiring is largely limited to meet a requirement of high-quality image printing.
Although one or two semiconductor laser elements per color are conventionally used, four semiconductor laser elements are becoming necessary to meet the requirement of high-quality image printing. With the increase in the number of semiconductor laser elements, a larger number of parts and wiring should be arranged on the laser circuit substrate. On the other hand, the optical axis of a laser beam is stabilized by fixing the laser circuit substrate to a metal housing around it. For this reason, when a substrate with a larger size is adopted in order to increase the degree of freedom of wiring, the vibration resistance decreases, so the optical axis readily shifts. The requirement of high-quality image printing cannot then be satisfied.
Hence, it is becoming very difficult for the laser circuit substrate to take the wiring form which employs wiring 4 and wiring 5 having the same length, or makes their length as small as possible.
In the wiring form shown in FIG. 9, the value of an impedance Z6 of the light-emitting circuit 6 is slightly different from that of an impedance Z10 of the compensation circuit 10. This difference takes a value small enough at a driving frequency of about 10 MHz and does not cause serious problems. However, it readily causes problematic noise currents under the present circumstance in which high-speed driving up to a driving frequency of about 60 MHz is desired. The requirement of printing at a high driving frequency cannot be sufficiently satisfied.